Porous ceramic structure for carbon dioxide capture

ABSTRACT

In one inventive concept, a product includes a three dimensional ceramic structure having an open cell structure with a plurality of pores, wherein the pores connect through the ceramic structure from one side of the ceramic structure to an opposite side of the ceramic structure.

The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC for the operation of Lawrence Livermore National Laboratory.

FIELD OF THE INVENTION

The present invention relates to porous ceramic materials with controlled, tunable porosity, and more particularly, this invention relates to porous ceramic particularly useful for a dual phase, molten salt membrane for carbon dioxide capture.

BACKGROUND

Porous ceramic is a porous material with nanoscale porosity that may withstand extreme environmental conditions, for example, high temperatures, high pH, low pH, etc. Thus, application of porous ceramic material can range from gas and liquid filtration, catalyst supports, thermal insulators, biomedical application, etc. A conventional method of producing porous ceramic material with control of the pore size and density involves combining ceramic particles with sacrificial porogens in the green body. The porogens may be burned out during sintering to form voids in the final part. Some approaches to form ceramic parts have involved combining ceramic nanoparticles with inexpensive organic powders such as sucrose or rice flour as the sacrificial porogen, followed by casting discs in a powder press. However, this methodology limits throughput; the powder press can only make one disc at a time, and the process requires several hours at high pressure. Thus, in addition to the overall throughput being insufficient the method using sacrificial porogens is cost prohibitive to produce the volume of ceramic parts needed for large scale applications. An affordable and scalable process for forming porous ceramics with tunable and fine control of microporosity remains elusive.

Carbon dioxide (CO₂) separation from flue gas of fossil fuel burning power plants and other large point sources has become of particular interest as greenhouse gas emissions, and particularly CO₂ emissions has become a large scale concern in regards to environmental health as a contributor to global climate change.

One approach to limit the rise of CO₂ in the atmosphere is to capture the CO₂ at the source of emission and either sequester the CO₂ underground to keep it out of the atmosphere or utilize CO₂ as feedstock to make useful materials. This process known as carbon capture and sequestration (CCS) works best for large, fixed sources of CO₂ emissions, such as power plants and industrial facilities. Today, emissions from large point sources constitute over half of the total carbon emissions in the United States.

Fossil fuel power plants are the single largest source of anthropogenic carbon emissions world-wide. Typical fossil fuel power plants produce CO₂ as a relatively minor constituent in a mixture of exhaust gasses consisting mainly of N₂ and H₂O. For efficient storage, the CO₂ is separated from the mixture of exhaust gases (the carbon capture (CC) step), which can be challenging due to the dilute concentration of CO₂ and its low reactivity.

A current technology for carbon capture is liquid sorbent amine gas treatment. In this process, flue gas that has cooled to nearly ambient temperature (<50° C.) is bubbled through a solution containing basic amines (typically monoethanolamine, MEA), which react with the acidic CO₂ forming an acid-base adduct (e.g. carbamates). When the amine-containing solution becomes saturated with CO₂, it is pumped to a different reactor where it is heated above 100° C. to release the CO₂ for sequestration and regenerate the sorbent. However, carbon capture using amine gas treatment suffers from a number of drawbacks including low carbon dioxide loading capacity (kg CO₂ absorbed per kg absorbent), high equipment corrosion rate, and sorbent degradation by various gases (e.g. SO₂, NO₂, HCl, HF, O₂, etc) in flue gas. Moreover, the single largest issue with amine gas treatment is the extremely high energy needs for the regenerating the sorbent. For example, regenerating the sorbent may consume from 25% to as much as 40% of the total energy output of the power plant. Furthermore, an amine system adds an infrastructure to the power plan that about doubles the cost of a building the power plant.

Membrane gas separation may be a desirable alternative to amine gas treatment. Membrane separation may use much less energy than amine gas separation because membrane gas separation process is spontaneously driven by the difference in CO₂ partial pressure on either side of the membrane. Furthermore, membrane separation does not have a regeneration step.

Recent efforts toward developing highly selective polymer membranes for CO₂ separation, however, have drawbacks. Polymer membranes operate at low temperatures (typically <100° C.), which lowers the rate of CO₂ diffusion across the membrane. Very thin membranes may overcome the slow diffusion of CO₂, but the thinness of the membrane may impact mechanical properties and increase cost. In addition, since polymer membranes separate flue gas constituents based on the difference in their diffusion rates, the membranes may not be perfectly selective for CO₂.

In contrast to polymer membranes, dual phase membranes consist of a liquid phase supported by a porous solid phase in which the liquid is held in the pores by capillary force. For CO₂ separation, recent studies of dual phase membranes include a mixture of molten carbonate salts as the liquid phase.

Early approaches to dual-phase membranes for CO₂ separation with molten carbonate used a stainless steel support to transport carbonate (CO₃ ²⁻) and electrons in opposite directions for selective permeation of CO₂ at high temperatures, higher than 450° C. However, these methods proved undesirable because the stainless steel support was susceptible to oxidation under the CO₂/O₂ atmosphere. Thus, applications of CO₂ separation turned to porous ceramic materials instead of stainless steel to support molten carbonate.

Studies have explored mixed metal oxides to synthesize carbonate-ceramic dual-phase membranes for CO₂ separation at high temperatures. Moreover, using an oxide-conducting ceramic support eliminates the need of electrodes. However, although oxidation of the metal support can be avoided under the CO₂/O₂ atmosphere, CO₂ permeance in the dual phase membranes of these systems tends to be less than desirable.

Better CO₂ permeance has been demonstrated in CeO₂ supported dual-phase membranes infilled with molten carbonate at 550° C. However, for acceptable CO₂ flux, these dual phase membranes operate at temperatures above 650° C., which is higher than the temperature of the flue gas present in the combustion turbine or boiler. The separation is rate-limited by oxide (O²⁻) transport in the solid ceramic phase, which reacts with CO₂ to form carbonate. At temperatures below 650° C. oxide transport is very slow in the opposite direction through the solid phase.

It would be desirable to create a membrane system that could operate at temperatures of 300 to 650° C., where the minimum separation energy may be theoretically as low as 0.2 GJ/ton CO₂, which would be approximately 10 to 15 times less energy than the conventional sorbent-based amine gas treatment.

Accordingly, it would be beneficial to provide systems and techniques for separating carbon dioxide gas from emissions sources such as flue gas of fossil fuel power plants that are capable of operating at the incipient temperature of the flue gas as emitted from the combustion process with a sufficient energy (e.g. temperature) to effectively separate the carbon dioxide without consuming substantial power from the source and without associated infrastructure costs incurred by the existing techniques and systems described above.

It would be desirable to develop a porous ceramic material with tunable porosity with the ability to retain molten salts at extreme temperatures for an extended period of time.

SUMMARY

In one inventive concept, a product includes a three dimensional ceramic structure having an open cell structure with a plurality of pores, wherein the pores connect through the ceramic structure from one side of the ceramic structure to an opposite side of the ceramic structure.

In another inventive concept, a method of forming a porous ceramic structure includes combining metal oxide nanoparticles and a polymer to create a mixture, forming a body of the mixture, curing the formed body for crosslinking the polymer, heating the formed body to a first temperature for removing the polymer from the formed body, and heating the formed body to a second temperature for sintering the formed body of the metal oxide nanoparticles, thereby creating a porous ceramic structure having an open cell structure with a plurality of pores, where the pores connect through the ceramic structure from one side of the ceramic structure to an opposite side of the ceramic structure.

Other aspects and advantages of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic diagrams of a reversible CO₂ absorption by molten hydroxides above 250° C. in the presence of water vapor, according to inventive concepts described herein.

FIG. 2A is a schematic drawing of a porous ceramic structure, according to inventive concepts described herein.

FIG. 2B is a schematic drawing of a magnified view of circle 2B of FIG. 2A

FIG. 3 is a flow chart of a method according to inventive concepts described herein.

FIG. 4A is a schematic drawing of a portion of a device, according to inventive concepts described herein.

FIG. 4B is an image of three-dimensional porous ceramic structures, according to inventive concepts described herein.

FIG. 5A is a schematic diagram of a direct ink writing process with a printing nozzle, according to inventive concepts described herein.

FIG. 5B is an image of a top down view of a log-pile three dimensional structure formed by direct ink writing. FIG. 5B includes a magnified view of a portion of the structure.

FIG. 5C is an image of a perspective view of a log-pile three dimensional structure formed by direct ink writing.

FIGS. 5D-5F are images of various perspective views of a cylindrical three dimensional structure formed by direct ink writing, according to inventive concepts described herein.

FIG. 6A is a schematic drawing of a process of projection micro-stereolithography.

FIG. 6B is an image of a three dimensional structure formed by projection micro-stereolithography, according to various embodiment described herein. FIG. 6B includes a magnified view of a portion of the three dimensional structure.

FIG. 7A is a scanning electron micrograph (SEM) image of 3Y-TZP 40 nm nanoparticles after sintering at low magnification (1 μm).

FIG. 7B is a SEM image of 3Y-TZP 40 nm nanoparticles after sintering at high magnification (10 μm).

FIG. 7C is a SEM image of a 60 wt % 3Y-TZP porous membrane after sintering at low magnification (1 μm), according to inventive concepts described herein.

FIG. 7D is a SEM image of a 60 wt % 3Y-TZP porous membrane after sintering at high magnification (10 μm), according to inventive concepts described herein.

FIG. 8A is a SEM image of a 60 wt % 3Y-TZP porous membrane infilled with KOH, according to inventive concepts described herein.

FIG. 8B is a SEM image of a fractured edge of a 60% 3Y-TZP porous membrane infilled with KOH, according to inventive concepts described herein.

FIG. 9A is a SEM image of a 75 wt % 3Y-TZP porous membrane, according to inventive concepts described herein.

FIG. 9B is a SEM image of a 70 wt % 3Y-TZP porous membrane, according to inventive concepts described herein.

FIG. 9C is a SEM image of a 65 wt % 3Y-TZP porous membrane, according to inventive concepts described herein.

FIG. 9D is a SEM image of a 60 wt % 3Y-TZP porous membrane, according to inventive concepts described herein.

FIG. 9E is a SEM image of a 55 wt % 3Y-TZP porous membrane, according to inventive concepts described herein.

FIG. 10A is a high magnification SEM image of a 70 wt % 3Y-TZP porous membrane formed at a final heating step of 1100° C. for 15 hours, according to inventive concepts described herein.

FIG. 10B is a low magnification SEM image of a 70 wt % 3Y-TZP porous membrane formed at a final heating step of 1100° C. for 15 hours, according to inventive concepts described herein.

FIG. 10C is a high magnification SEM image of a 70 wt % 3Y-TZP porous membrane formed at a final heating step of 1200° C. for 3 hours, according to inventive concepts described herein.

FIG. 10D is a low magnification SEM image of a 70 wt % 3Y-TZP porous membrane formed at a final heating step of 1200° C. for 3 hours, according to inventive concepts described herein.

FIG. 10E is a high magnification SEM image of a 70 wt % 3Y-TZP porous membrane formed at a final heating step of 1300° C. for 5 hours, according to inventive concepts described herein.

FIG. 10F is a low magnification SEM image of a 70 wt % 3Y-TZP porous membrane formed at a final heating step of 1300° C. for 5 hours, according to inventive concepts described herein.

FIG. 11 is plot of the chemical stability of (LiKNa)OH infilled in a porous 70 wt % 3Y-TZP membrane over 1000 hours, according to inventive concepts described herein.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless otherwise specified.

As also used herein, the term “about” when combined with a value refers to plus and minus 10% of the reference value. For example, a length of about 1 μm refers to a length of 1 μm±0.1 μm.

It is also noted that, as used in the specification and the appended claims, wt % is defined as the percentage of weight of a particular component is to the total weight of the mixture. Moreover, mol % is defined as the percentage of moles of a particular component is to the total moles of the mixture.

The following description discloses several preferred inventive concepts of porous ceramic structure for carbon dioxide capture and/or related systems and methods.

In one general inventive concept, a product includes a three dimensional ceramic structure having an open cell structure with a plurality of pores, wherein the pores connect through the ceramic structure from one side of the ceramic structure to an opposite side of the ceramic structure.

In another general inventive concept, a method of forming a porous ceramic structure includes combining metal oxide nanoparticles and a polymer to create a mixture, forming a body of the mixture, curing the formed body for crosslinking the polymer, heating the formed body to a first temperature for removing the polymer from the formed body, and heating the formed body to a second temperature for sintering the formed body of the metal oxide nanoparticles, thereby creating a porous ceramic structure having an open cell structure with a plurality of pores, where the pores connect through the ceramic structure from one side of the ceramic structure to an opposite side of the ceramic structure.

A list of acronyms used in the description is provided below.

-   -   3D Three dimensional     -   Al₂O₃ Alumina, aluminum oxide     -   AM Additive manufacturing     -   BET Brunauer-Emmett-Teller theory surface area calculation     -   C Celsius     -   CeO₂ Cerium(IV) oxide     -   12Ce-TZP 12 mol % CeO₂ tetragonal ZrO₂ polycrystal     -   CCS Carbon capture and storage     -   CO₂ Carbon dioxide     -   g gram     -   GJ gigajoule     -   HEPA High efficiency particulate air     -   IL Ionic liquids     -   KOH Potassium hydroxide     -   LiOH Lithium hydroxide     -   (LiNaK)OH Mixture of equimolar amounts of LiOH/NaOH/KOH     -   MEA monoethanolamine     -   MECS Microencapsulated carbon sorbents     -   mg mg     -   ml milliliters     -   mol % mole percent     -   Na₂CO₃ Sodium carbonate     -   NaOH Sodium hydroxide     -   nm nanometer     -   PEGDA Polyethylene glycol diacrylate     -   SEM Scanning electron micrograph     -   SiO₂ Silicon oxide     -   SnO₂ Tin oxide     -   TGA Thermogravitational analysis     -   TiO₂ Titanium dioxide     -   μm micron, micrometer     -   UV Ultraviolet     -   wt % weight percent     -   3Y-TZP 3 mol % Y₂O₃ tetragonal ZrO₂ polycrystal     -   8YSZ 8 mol % Y₂O₃ fully stabilized ZrO₂     -   ZrO₂ Zirconium oxide

One approach to lower the temperature of operation for dual-phase membranes while improving overall CO₂ flux may include transporting the oxide ion (O²⁻) in the liquid phase, which could be accomplished by replacing the molten carbonate with molten hydroxide. However, replacing the molten carbonate phase with molten hydroxide may result in an irreversible reaction of CO₂ with hydroxide. Recently, research into molten carbonate fuel cells has independently reported that CO₂ absorption by molten hydroxides may be reversible above 250° C. in the presence of sufficient water vapor (steam).

In order to support molten hydroxides, it is desirable for the support material to be chemically (e.g. resistant to strong base, humid conditions, etc.) and thermally robust (e.g. withstand up to ˜1000° C.). Moreover, it is desirable that the support material has a pore size and morphology to retain the molten phase in the pores against modest differential pressure. Inventive concepts described herein demonstrate a method for producing porous ceramic materials with tunable density and pore morphology. Some of the methods described herein involve using a sacrificial polymer template that result in a three dimensional (3D) ceramic structure that may be chemically and thermally stable. In addition, the formed 3D ceramic structure may support infill materials, such as highly corrosive molten liquid salts (e.g. molten sodium hydroxide) at temperatures from 250° C. to 650° C. In some approaches, the formed 3D ceramic structure may support infill ionic liquids that may be liquid at temperatures from 250° C. to about room temperature (e.g. between about 20° C. and 25° C.). In other approaches, the formed 3D ceramic structure may support infill ionic materials at temperatures as high as about 1000° C.

Moreover, aspects of inventive concepts described herein produce high surface area material that can withstand harsh chemical environments and high temperature environments. In some approaches, a ceramic membrane is formed that may support aqueous carbonate and/or ionic-liquid solutions. The resulting ceramic membrane may be used to support a liquid phase that is confined in the pores by capillary forces.

Inventive concepts described herein demonstrate a thermally and chemically stable ceramic support may facilitate thermal regeneration of the sorbent, and may not significantly degrade at elevated temperatures and/or in the presence of corrosive chemicals. A method is described for forming a porous ceramic structure. In some approaches, the porous ceramic structure may be used for CO₂ capture.

Ceramic supports may be produced in arbitrary shape and/or size by casting into molds. In other embodiments, ceramic supports may be produced by additive manufacturing (AM) methods for greater selectivity of morphology and further optimized CO₂ absorption performance. Other methods of formation may include extrusion, roll pressing, etc.

FIG. 1A is a schematic drawing of a system 100 of a combined-cycle natural gas power plant. As shown, the exhaust from a combustion turbine 102, including primarily CO₂, N₂, and water vapor, flows into a heat exchanger 104. The heat exchanger 104 heats water producing steam 108 that powers a steam turbine 110 and which is then condensed to liquid water 112 in a closed loop. The heat exchanger 104 removes heat from the flue gas (which exits the combustion turbine at 300° C. to 650° C.) cooling the gases for release 106 at less than 50° C. Conventional carbon capture methods involve capturing the carbon at the release 106 stage. However, cold CO₂ separation at temperatures lower than 50° C. with atmospheric pressure less than 1.5 atm tend to be energetically inefficient. It would be desirable to develop a separation membrane process that could operate along with or inside the heat exchanger 104 when the gases are at high temperatures of 300° C. to 650° C. with a pressure of 5 to 20 atm.

FIG. 1B shows a schematic diagram of a dual phase membrane process 150 of absorption and desorption of CO₂ using a dual phase membrane 118 that includes molten hydroxide 116 in a porous support 114. The hot flue gas includes N₂, CO₂, etc. that upon contacting the membrane 118, the CO₂ may be selectively absorbed by the molten hydroxide 116 according to Equation 1. 2 KOH+CO₂→K₂CO₃+H₂O  Equation 1

The CO₂ absorbed with the molten hydroxide 116 in the porous support 114 may be transported across the membrane 118 and desorbed in a desorption step of Equation 2, thereby demonstrating a reversible reaction of the CO₂ with molten hydroxide 116 of the porous support 114. K₂CO₃+H₂O→2 KOH+CO₂  Equation 2

The desorption step of Equation 2 may be initiated by low CO₂ pressure and steam to release CO₂ from the membrane on the sweep side of the porous support 114.

In more approaches, and preferable in some applications to reduce energy consumption associated with separation of offensive and/or acidic gases from gas mixtures such as flue gas, the presently disclosed inventive separation membranes and processes may accomplish separation without applying a pressure gradient via use of a sweep gas. For instance, in one embodiment water vapor may be passed across the permeate side of the membrane (opposite the side where offensive and/or acidic gases are present in high concentration, e.g. in the flue). Optionally, but advantageously, the sweep gas may apply pressure to the permeate side of the membrane.

Using a sweep gas is an attractive approach for several reasons. First, the water pushes the equation (2) equilibrium over to favor CO₂ release on the permeate side of the membrane. Second, CO₂ separates at a higher rate than without sweep gas utilization, because the sweeping action maintains a larger CO₂ gradient across the membrane than a vacuum, with less energy. Third, if the steam pressure applied to the permeate side of the membrane is approximately equal to the pressure applied to the membrane by the gas mixture (e.g. flue side), there is no difference in total pressure across the membrane, which significantly relaxes the materials challenges for capillary action (pore size) and mechanical strength. Fourth, steam is already present and available in various suitable applications, such as power plants (where steam is used in the heat exchanger), obviating the need to provide external sources of steam and further improving energy efficiency of separation. Fifth, water vapor can be easily separated from the CO₂ after gas separation (e.g. via condensation), which further allows advantageous harvesting of heat from the water vapor. Accordingly, a faster movement of acid gases across the membrane may desirably minimize infrastructure quantity and cost of the overall separation process/solution.

FIGS. 2A and 2B depict schematic drawings of perspective views of a product 200 that includes a porous ceramic structure, in accordance with one embodiment. As an option, the present product 200 may be implemented in conjunction with features from any other embodiment listed herein, such as those described with reference to the other FIGS. Of course, however, such product 200 and others presented herein may be used in various applications and/or in permutations which may or may not be specifically described in the illustrative embodiments listed herein. Further, the product 200 presented herein may be used in any desired environment.

According to inventive concepts described herein, a product 200 includes a porous ceramic structure 202. FIG. 2B is a schematic drawing of the circle 2B of ceramic structure 202. FIG. 2B depicting a magnified portion of the porous ceramic structure 202, shows the porous ceramic structure 202 may have an open cell structure with a plurality of pores 206, where the pores 206 may connect through the ceramic structure 202 from one side of the ceramic structure to an opposite side of the ceramic structure.

In various approaches, the ceramic structure may include alumina (Al₂O₃), 8 mol % yttria fully stabilized zirconia (8 mol % Y₂O₃-doped ZrO₂ (8YSZ)), 3 mol % yttria (tetragonal zirconia polycrystal (3 mol % Y₂O₃-doped ZrO₂ (3Y-TZP)), CeO₂, 12 mol % CeO₂ tetragonal ZrO₂ polycrystal (12Ce-TZP), ZrO₂, SiO₂, TiO₂, SnO₂, etc. In preferred approaches, the ceramic structure may include 3 mol % Y₂O₃-doped ZrO₂. In other preferred approaches, the ceramic structure may include 8 mol % Y₂O₃-doped ZrO₂.

The porous ceramic structure 202 may be formed in any suitable configuration. While the configuration shown in FIG. 2A is characterized by a circular configuration, other shapes may be implemented without departing from the scope of the present disclosures. In various approaches, a variety of other geometries in various dimensions may be formed.

In some approaches, the porous ceramic material may have a geometry represented by multiple structures such that the resulting porous ceramic structure may have a diameter of about 1 to about 10 cm but could be smaller or larger. In some approaches, the porous ceramic material may be extruded, milled, or molded into shapes (e.g. cylinder, cube, etc.), etc. In some approaches, the formed structures of porous ceramic material may have pores, ligaments, features, etc. having a size of about 100 to about 200 nm,

According to inventive concepts described herein, the porous ceramic structure may function as a support for a liquid phase, where the liquid phase may be contained in the pores using capillary pressure. Preferably, the average pore size is such that the liquid phase substantially remains in the pores under conditions corresponding to the intended use of the resulting product. Furthermore, the porous ceramic structure infilled with a liquid may function as a membrane to selectively transport materials (e.g. CO₂) from one side of the membrane to the other. In some approaches, the pores may be a size that holds a liquid in a pore against an arbitrary pressure applied across the membrane. For example, smaller pores may withstand higher pressure applied across a membrane; however, smaller pores may result in a reduced capacity for liquid and a slower mass transport rate.

In some approaches, the porous ceramic structure may have pores having an average diameter sufficient to retain liquid by capillary action. In some approaches, the porous ceramic structure may have pores having an average diameter in a range of about 50 nm to about 500 nm, but the diameter of the pores may be lower or higher. In some approaches, the porous ceramic structure may have pores having a diameter in a range from approximately 75 nm to about several (e.g. 10) microns. As will be understood by persons having ordinary skill in the art upon reading the present descriptions, larger pore size is desirable to facilitate rapid mass transport through the membrane, but pores must be sufficiently small to retain the molten hydroxide phase within the matrix when subjected to the pressure gradient generated within the separation environment. In preferred approaches, the porous ceramic structure may have pores having an average diameter in a range of about 50 nm to about 300 nm.

Furthermore, pores may have an average diameter in a range from about 10 nm to about 1 mm, from about 100 nm to about 100 μm, from about 100 nm to about 10 μm, from about 200 nm to about 5 μm, or in a range from about 200 nm to about 1 μm, in various embodiments. As will be appreciated by skilled artisans upon reading the present disclosures, the pore size is critical to retention of hydroxide in pores of the porous support structure under a given pressure gradient.

In some embodiments, the density of the porous ceramic structure may be in a range of about 20% to about 50% of a density of a solid ceramic form, but could be higher or lower. In a preferred approach, the density of the porous ceramic structure may be in a range of about 20 wt % to about 50 wt % of a density of a solid ceramic form having the same composition as the porous ceramic structure. In some approaches, sintering the structure at higher temperature conditions may result in a density of the porous ceramic structure in a range of about 45 wt % to about 85 wt % of a density of a solid ceramic form.

Looking to FIGS. 2A and 2B, in some approaches, the porous ceramic structure 202 may include a molten hydroxide 204 in the pores 206, where the molten hydroxide 204 is retained in the pores 206 by capillary action. In some approaches the porous ceramic structure may include a molten potassium hydroxide. In other approaches, the molten hydroxide may be a mixture of LiOH, NaOH, and KOH. In various approaches, the porous ceramic structure including a molten hydroxide may be used for capturing CO₂. In some approaches, the product 200 may include a porous ceramic structure 202 with hydroxide 204 disposed in the continuous pore system defined by the voids of the porous support.

In inventive concepts described herein, a porous ceramic structure may retain molten hydroxide in extreme conditions with long term stability, such as high temperatures over an extended period of time. In some approaches, the ceramic structure may have physical characteristics that enable the ceramic structure to retain the molten hydroxide in the pores after exposure to temperatures at about 400° C. for at least 100 hours under nitrogen gas (N₂). In some approaches, the ceramic structure may have physical characteristics that enable the ceramic structure to retain the molten hydroxide in the pores after exposure to high temperatures (e.g. 400° C.) for at least 1000 hours under nitrogen gas (N₂). In some approaches, the ceramic structure may have physical characteristics that enable the ceramic structure to retain molten hydroxide in the pores at high temperatures (e.g. 400° C.) for as long as 17,520 hours (i.e. 2 years).

According to inventive concepts described herein, the ceramic structure may have physical characteristics that enable the ceramic structure to retain the molten hydroxide in the pores after at least twenty heating and cooling cycles, wherein the heating may be to about 400° C. and the cooling may be to at least room temperature (room temperature may be between 20° C. to 25° C.). In some approaches, the porous ceramic structure may retain molten potassium hydroxide during greater than 40 heating and cooling cycles.

FIG. 3 shows a method 300 for forming a porous support ceramic membrane. As an option, the present method 300 may be implemented to construct structures such as those shown in the other FIGS. described herein. Of course, however, this method 300 and others presented herein may be used to form structures which may or may not be related to the illustrative concepts listed herein. Further, the methods presented herein may be carried out in any desired environment. Moreover, more or less operations than those shown in FIG. 3 may be included in method 300. It should also be noted that any of the aforementioned features may be used in any of the embodiments described in accordance with the various methods.

As shown in FIG. 3, step 302 of method 300 involves combining metal oxide nanoparticles and a polymer to create a mixture. Many types of metal oxides can be used. Metal oxides, for example but not limited to, alumina (Al₂O₃), 8 mol % yttria fully stabilized zirconia (8 mol % Y₂O₃-doped ZrO₂ (8YSZ)), 3 mol % yttria (tetragonal zirconia polycrystal (3 mol % Y₂O₃-doped ZrO₂ (3Y-TZP)), CeO₂, 12 mol % CeO₂ tetragonal ZrO₂ polycrystal (12Ce-TZP), ZrO₂, SiO₂, TiO₂, SnO₂, etc. may be obtained in nanoparticle form from commercial sources. In an exemplary approach, metal oxide nanoparticles may be yttrium-doped zirconium oxide. In some approaches, metal oxide nanoparticles may be synthesized using methods known by one skilled in the art. In some approaches, any ceramic particles that can be fused by sintering may be used. For example, but not limited to, ceramic particles such as nitrides, carbides, oxy-nitrides,—carbides, etc. may be used as nanoparticles in some approaches.

In some approaches, metal oxide nanoparticles may have any conceivable size for nanoparticles. In some approaches, the size of the nanoparticles may be arbitrary, e.g. the size may be defined by ability of the nanoparticles to form homogeneous slurries, by the desired pore size of the resulting ceramic structure for a given application, etc.

In preferred approaches, a majority of the nanoparticles have a maximum dimension smaller than 1000 nm. In some approaches, metal oxide nanoparticles may have an average size in the range from about 20 nm to about 600 nm, but the size may be smaller or larger. In some approaches, larger nanoparticles may diminish nanoporosity of the porous ceramic structure. In some approaches, one mixture may include nanoparticles of different sizes.

The polymer may be a polymer resin. The polymer resin may generally include any cross-linkable polymer. The polymer resin may be an acrylate that can be cross-linked using a thermal initiator (e.g. a cross-linking agent), for example but not limited to, polyethylene glycol diacrylate (PEGDA) plus a thermal initiator (e.g. 3 wt % Luperox 231). In other approaches, the polymer resin may be an acrylate that may be cross-linked using an ultraviolet (UV) light-activated initiator.

Moreover, the polymer resin may include other materials.

The ratio of metal oxide nanoparticles in the polymer resin of the mixture may vary depending, for example, on the intended use, desired porosity, etc. In some approaches, the concentration of metal oxide nanoparticles in polymer resin of the mixture may be in the range of about 50 wt % to about 80 wt % metal oxide for ceramic loading. The concentration of metal oxide nanoparticles may depend on the type of nanoparticles and/or metal oxides in the metal oxide nanoparticles. For example, if the ratio results in a dilute mixture of nanoparticles in polymer resin, the particles may settle out of the solution, the final porous ceramic structure may not have a final density high enough for mechanical strength, etc. Moreover, if the ratio results in a concentrated mixture of nanoparticles in polymer resin, the mixture may be too difficult to process, and not have an appropriate viscosity, e.g. the mixture may resemble clay rather than flowable, viscous liquid usable for molding.

The physical characteristics of the porous ceramic membrane, for example, but not limited to density, pore morphology, mechanical properties, etc., may be tuned by varying the ratio of metal oxide nanoparticles to polymer resin. In some approaches, the process conditions may be particularly selected depending on the ratio of metal oxide nanoparticles to polymer resin, as would be appreciated by one skilled in the art upon reading the present description.

In various approaches to step 302 of method 300, the mixture may be combined using techniques known in the art, for example but not limited to, sonication, resonant acoustic mixing (RAM), vortex, etc.

Step 304 of method 300 includes blending the mixture of metal oxide nanoparticles and a polymer with a conventional cross-linking agent into a homogenous mixture. In some approaches, the cross-linking agent may be a liquid. In other approaches, the cross-linking agent may be a solid. In some approaches, the cross-linking agent may be a thermal initiator. In other approaches, the cross-linking agent may be a photo-initiator, e.g. 2-hydroxy-2-methylpropiophenone. In some approaches, the cross-linking agent blended in the homogenous mixture may be Luperox®.

In some approaches, the mixture of metal oxide nanoparticles in polymer resin may be blended into homogenous suspension slurry of nanoparticles to ensure uniform density of nanoparticles throughout the composite. In some approaches the mixture may be blended by sonication. In other approaches, the mixture may be blended using a mechanical mixer, for example, a resonant acoustic mixer.

Step 306 of method 300 involves forming a body of the mixture. In some approaches, step 306 involves forming a body by using the blended slurry ceramic mixture from step 304 as a feedstock for various additive manufacturing (AM) processes.

In some approaches of step 306 the body may be formed by pouring the mixture into a mold (e.g. cast, die, template, pattern, etc.) to create a casting of the mixture of metal nanoparticles and polymer. In some approaches, the homogenous slurry ceramic mixture may be cast into free standing 3D shapes, e.g. cylindrical shapes, tubular shapes, etc. A device (e.g. an extruder) may allow formation of a shape defined by extruder nozzle, die, etc. For example, FIG. 4A is a schematic drawing of a portion of an extruder 402 that forms a cylindrical shape defined by the nozzle 404. FIG. 4B shows several free-standing tubes formed from an extruded slurry mixture of 70 wt % 3Y-TZP in a range of several centimeters in length.

In some approaches, as shown in FIGS. 5A through 5F, the blended slurry ceramic mixture may be used as an ink for direct ink writing. FIG. 5A is a schematic drawing of an apparatus 502 with a nozzle 504 used in direct ink writing in which the blended slurry mixture may be used as an ink 506 extruded from the nozzle 504. The apparatus 502 moves in three dimensions (as shown in arrows x, y, and z) to form a 3D structure 508 with the extruded ink 506. As an example only, and in no way meant to be limiting, a blended slurry of 70 wt % 8 mol % Y₂O₃—ZrO₂ (8YSZ) with PEGDA may be used as an ink to form a log-pile 3D structure as shown in FIG. 5B. A top down view of the structure is shown in FIG. 5B with a magnified view of a portion depicting the porosity of the 3D structure. FIG. 5C is a perspective view of the log-pile 3D structure formed by direct ink writing.

FIGS. 5D-5F show the formation of a cylindrical 3D structure (e.g. a tube) using the blended slurry mixture as an ink in direct ink writing. The nozzle as shown in FIG. 5A extrudes the blended slurry mixture in a continuous cyclic pattern to form a hollow cylinder 3D structure.

In other approaches, the blended slurry ceramic mixture may be a UV-curable polymer resin used in projection micro-stereolithography in which a patterned part of porous ceramic structure may be formed. FIG. 6A is a schematic drawing of a process 600 of forming a 3D structure 614 using projection micro-stereolithography with a UV-curable polymer resin. Briefly, and as an example but not meant to be limiting in any way, a 3D CAD model 602 may be used for a digital mask 604 to shine a pattern of light via beam delivery 606. A pattern may be projected through a projection lens 608 onto a substrate 616 in a bath of UV-curable ceramic/resin 612. A 3D structure 614 may be formed as the projected light reacts with the UV-curable ceramic/resin 612, and subsequent layers of the forming 3D structure 614 are formed as the substrate 616 on the elevator 610 moves in the z-direction. From the UV-curable ceramic/resin 612 a polymer part 618 may be formed.

FIG. 6B shows an example, that is not meant to be limiting, of a polymer-ceramic composite part formed from a blended slurry mixture of 60 wt % 3Y-TZP with PEGDA using an additive manufacturing technique of projection micro-stereolithography (before sintering and polymer burn-out). The magnified view 620 shows the porosity of a portion of the 3D structure after sintering.

Looking back to method 300 in FIG. 3, following formation of body with the blended slurry mixture, step 308 that involves curing the formed body. The process of curing the formed body may depend on the polymer and cross-linking agent used in creating the mixture. In some approaches, the homogenous mixture of metal oxide nanoparticles, polymer resin, and cross-linking agent is formed in the mold and then cured to induce cross-linking and to produce parts of arbitrary dimensions. In other approaches, the homogenous mixture of metal oxide nanoparticles, polymer resin, and cross-linking agent in the mold may be cured by UV light. In another approach, the slurry ceramic mixture may be printed into a 3D structure having a desired shape, and then cured.

Step 310 of method 300 includes heating the formed body to a first temperature for removing the polymer from the formed body. Step 310 may be performed at atmospheric pressure and under air. In some approaches of step 310, the heating the formed body to a first temperature may be stepwise heating process where the formed body of cured mixture may be heated to one temperature at a defined rate and then held at the temperature for a defined amount of time. The temperature may be raised at a defined rate to a higher hold temperature for a defined amount of time. These steps may be repeated. A final step of the first heating process may include cooling at a set rate. In some approaches, the parameters (e.g. temperature, rate of temperature change, timing, atmosphere, etc.) for step 310, a first heating process for removing the polymer, may depend on the materials, e.g. metal oxide nanoparticles, polymer resin, etc. In some approaches, the molded form may be heated to 400° C. for two hours to remove the polymer.

Step 312 of method 300 includes heating the formed body to a second temperature for sintering the formed body of metal oxide nanoparticles. In some approaches, the second temperature for sintering may be different from the first temperature for removing the polymer. In some approaches, step 312 sinters the formed body of metal oxide nanoparticles, thereby creating a porous ceramic structure having an open cell structure with a plurality of pores, where the pores connect through the ceramic structure from one side of the ceramic structure to an opposite side of the ceramic structure. In some approaches, the metal oxide nanoparticles may be sintered to form a monolithic ceramic piece with pores.

In some approaches, step 312 of heating the formed body for sintering may include two or more heating steps. In some approaches of step 312, the heating the formed body to a second temperature may be stepwise heating process where the formed body of cured mixture with the polymer removed may be heated to one temperature at a defined rate and then held at the temperature for a defined amount of time. Then the temperature may be raised at a defined rate to a higher hold temperature for a defined amount of time. These steps may be repeated. A final step of the second heating process may include cooling at a set rate. The absence of polymer in the resulting sintered porous ceramic structure may be confirmed by thermogravimetric analysis (TGA).

In some approaches, the parameters (e.g. temperature, rate of temperature change, timing, atmosphere, etc.) step 312 involving the sintering process may depend on metal oxide nanoparticles as would be appreciated by one skilled in the art upon reading the present description. In some approaches, the optimal sintering temperature may depend on the metal oxide nanoparticles in the mixture. In some approaches, the second temperature for sintering may be in a range of about 1100° C. to about 1300° C. The duration of time for heating at the second temperature for sintering the formed structure may be in a range of about 3 hours to about 15 hours. As an example, but not to be limiting in any way, formed structures of metal oxide nanoparticles may be heated to 1100° C. for 15 hours to sinter the structures. For some metal oxide nanoparticles, higher sintering temperatures may result in higher coarsening of the particles such that the sintered structure may have lower surface area and larger pores.

In one embodiment, a formed cast of a cured mixture may be removed from a mold, followed by heating the formed cast to remove the polymer content and sintering the cast to create pores in the cast. The resulting porous ceramic structure may be used for a variety of purposes, for example, the structure may function as a filter. In other approaches, a porous ceramic structure may function as support material for catalysis applications. In yet other approaches, a porous ceramic structure may function as a growth apparatus in growth media for cell culture growth.

Porous Ceramic Structure Infilled with Molten Hydroxide

Method 300 may further include filling the porous ceramic structure with a material. In some approaches, the porous ceramic structure may be filled with a material that includes an inorganic base material. For membrane application, the infilling liquid may be an inorganic compound (e.g. base) or mixture of compounds that has a high solubility of CO₂, such as sodium hydroxide, potassium carbonate, sodium borate, sodium phosphate, any of many other inorganic solvents that are bases in the sense of acid-base reactions, etc.

In some approaches, the porous ceramic structure may be filled with an inorganic base material that includes a molten hydroxide. In some approaches, the porous ceramic structure may be filled with a molten hydroxide that includes molten potassium hydroxide. In an exemplary approach, the porous ceramic structure may be infilled with a mixture of molten hydroxides, e.g. (LiNaK)OH.

The hydroxide may be heated to the melting temperature of the hydroxide thereby resulting in a molten hydroxide. The porous ceramic structure may be filled with the molten hydroxide at a temperature that maintains the molten nature of the hydroxide.

In one exemplary approach, a molten mixture of alkaline hydroxide may be infilled in the porous ceramic structure by a method of dipping. For example that is not meant to be limiting, the hydroxide may be melted to a liquid, e.g. molten hydroxide, at 400° C. Then, the porous ceramic structure is dipped into the molten hydroxide and removed from the molten hydroxide. Residual excess molten hydroxide may solidify on the surface of the porous ceramic structure as the excess molten hydroxide cools on the surface. The excess molten hydroxide on the surface of the porous ceramic is removed by heating the porous ceramic structure infilled with molten hydroxide to a temperature in the range of about 400° C. to about 550° C. thereby allowing the excess molten hydroxide on the surface to drip off the structure surface.

In some approaches, a product for capturing carbon dioxide may include a mixture of molten hydroxides in the pores of the ceramic structure. A method of using the porous ceramic structure may be as a support membrane for a continuous process of separating gases that includes contacting the gases with a molten hydroxide retained in the pores of a porous ceramic structure.

A continuous process for separating gases may begin with forming a ceramic structure having an open cell structure with a plurality of pores, where the pores connect through the ceramic structure from one side of the ceramic structure to an opposite side of the ceramic structure. In some approaches, a porous ceramic structure may be acquired commercially, e.g., from a vendor practicing methods of fabricating porous ceramic structures as described elsewhere herein.

For gas separation at high temperatures, the formed porous ceramic structure may be a support for molten hydroxide, molten salt mixtures, etc. for functioning as a mobile phase for selective separation in a dual-phase membrane. In some approaches, the dual-phase, molten salt membrane using the porous ceramic structure may function at temperatures above 300° C.

In some approaches, a continuous process of separating gases may include contacting the gases with the molten hydroxide in the formed ceramic structure such that the dual-phase, molten salt membrane may function as a continuous operation of feed side/sweep side (e.g. permeate and retentate sides) for efficient CO₂ gas capture and release. The feed side of the dual-phase, molten salt membrane includes contacting a gas mixture with the membrane (e.g. molten hydroxide of the porous ceramic structure). The dual-phase, molten salt membrane may selectively react with the CO₂ of the gas mixture and transport the CO₂ across the membrane to the sweep side of the membrane thereby resulting in a pure stream of CO₂ on the sweep side. In various approaches, the dual-phase, molten salt membrane may be tuned to separations of specific gases by using different salt mixtures, sweep gas, operating conditions, etc.

As described in inventive concepts herein, contacting a CO₂ gas with molten potassium hydroxide retained in a ceramic structure may result in a CO₂ permeance greater than about 2.5×10⁻⁸ mol/m-s-Pa. Moreover, contacting a CO₂ gas with molten potassium hydroxide retained in a ceramic structure may result in a CO₂ permeability of greater than about 60 10⁻¹² mol/m-s-Pa.

In preferred approaches, a mixture of molten hydroxide (e.g. approximately equimolar amounts of NaOH/KOH/LiOH, or amounts in a range as described by the ratios provided below, in various embodiments) provides an exceptional ionic conductivity of 1 S/cm at 300° C. even though the viscosity thereof is in a range of approximately 2-3 centipoise. In another embodiment, the eutectic mixture may include alkali metal hydroxides according to a ratio of 4KOH:4NaOH:1LiOH. In another embodiment, the mixture may include alkali metal hydroxides according to a ratio of 1KOH:1NaOH:4LiOH.

In preferred embodiments, the porous ceramic structure is infilled with a eutectic mix of NaOH/KOH/LiOH because the eutectic mix of NaOH/KOH/LiOH has a lower melting point and higher solubility of CO₂ and carbonate ions CO₃ ²⁻ thereby allowing the molten hydroxide to remain liquid during a CO₂ separation process.

In one approach, porous ceramic structures infilled with molten hydroxide solution may be arranged in a multi-layer structure, stack, fixed packing, etc. to capture CO₂ from a mixed gas stream. Then the porous ceramic membrane structure may be treated with steam, in place or at another location, to de-gas, desorb, release, etc. the CO₂ from the membrane structure.

Porous Ceramic Structure Infilled with Sorbent Solution

In some approaches, the porous ceramic structure may be filled with sorbent solution that has a preferentially high solubility of CO₂ and a low solubility of other gas stream components such as nitrogen and oxygen. Solutions that are alkaline are desirable because CO₂ tends to ionize in an alkaline solution to form bicarbonate (HCO₃ ⁻) and carbonate (CO₃ ²⁻) The sorbent solution may be an amine such as MEA or other amine-based solutions that have high solubilities for CO₂.

In some approaches, the porous ceramic structure may be infilled with sorbent solution by soaking the porous ceramic structure in the sorbent solution. In other approaches, the porous ceramic structure may be infilled with sorbent solution by infusing the sorbent solution into the porous ceramic structure. In yet other approaches, the porous ceramic structure may be infilled with sorbent solution by penetrating (e.g. permeating) the structure with sorbent solution.

For gas separation at low temperatures, the formed porous ceramic structure may be support for sorbent solution/ionic liquids that absorb CO₂ at temperatures typically below 50° C. In some approaches, the sorbent (e.g. ceramic structure with sorbent solution) becomes saturated with CO₂ and then may be regenerated by heating above about 100° C. thereby releasing the CO₂.

In some approaches, the sorbent solution added to the porous ceramic structures may be an aqueous sorbent solution. In some embodiments, the solution may be an aqueous carbonate (for example, Na₂CO₃, K₂CO₃) for capture of CO₂ from mixed gas streams. In some approaches, the material added to the porous ceramic membrane may be an aqueous solution of sodium carbonate (Na₂CO₃), for example 20% Na₂CO₃ at room temperature, or the concentration of Na₂CO₃ may be higher at higher solubility temperatures.

In some approaches, the sorbent solution may be an ionic liquid that may be designed specifically for CO₂ capture from mixed gas streams. In some approaches, the porous ceramic support membrane may be filled with ionic liquids that have physical characteristics that are challenging to use in conventional process equipment, for example, high viscosity and/or phase changes, etc.

A sorbent of porous ceramic structures filled with sorbent solution may increase the rate of absorption of CO₂ compared to bulk sorbent solution, e.g., exposed in a tray. Without wishing to be bound by any theory, it is believed that the ceramic component of the sorbent provides a nano-confinement gas solubility effect that may increase the absorbent efficiency of the sorbent solution infilled in the porous ceramic structure by a factor of at least four, and in some cases by a factor of up to ten.

In some approaches, the sorbent process may be used to separate CO₂ in a batch, step-wise, etc. operation. In some approaches, after absorption of CO₂, the porous ceramic structures with sorbent solution may be heated to release the CO₂ from the sorbent solution. The recharged sorbent of porous ceramic structures infilled with sorbent solution may be used again to capture CO₂.

In one approach, porous ceramic structures infilled with sorbent solution may be arranged in a multi-layer structure, stack, fixed packing, sheets, tiles with a large footprint area, etc. to capture CO₂ from a mixed gas stream. Then the porous ceramic structure with CO₂-saturated sorbent solution may be heated, in place or at another location, to de-gas, desorb, release, etc. the CO₂ from the sorbent.

In another approach, small particles of porous ceramic structures (e.g. roughly spherical, crushed porous ceramic structures) infilled with sorbent solution may be arranged in a fluidized or packed bed system such that one bed would be for absorption of CO₂, followed by moving the particles to a second bed for desorption of the CO₂ at a different temperature. Without wishing to be bound by any theory, it is believed that the increased surface area of the sorbent as indicated by the plurality of small particles of porous ceramic structures in a sorbent solution provides faster rates of absorption/desorption of CO₂ compared to bulk sorbent solution of similar composition.

Experiments

Fabrication of Porous Ceramic Membrane

14 mm diameter, 1 mm thick porous support ceramic membrane disks were fabricated by mixing different amounts of metal oxide nanoparticles (3 mol % Y₂O₃-doped ZrO₂ (3Y-TZP, Tosoh USA, Inc, Grove City, Ohio) and PEGDA (Mn 575). After mechanical mixing (LabRAM, Butte, Mont.) during 30 mins, 3 wt % Luperox® 231 sold by Sigma-Aldrich (St. Louis, Mo., USA) was added to the slurry mixture. The homogenous mixture was cast into punched discs silicon rubber, covered with a slide, and thermally cured at 80° C. during 5 h.

The polymer-ceramic nanoparticles were then removed from the molds. The polymer-ceramic nanoparticle molds were heated in a box furnace under air to remove the polymer template, holding at 180° C. for 2 hours, 250° C. for 4 hours, 400° C. for 2 hours, heating rate 1° C./min. The ceramic nanoparticle form was then heated to sinter the ceramic nanoparticle forms into a porous membrane by holding at 800° C. for 4 hours and 1100° C. for 15 hours, heating and cooling rate 2° C./min. In parallel, 3Y-TZP nanoparticles were sintered under similar conditions as the polymer-ceramic nanoparticle molds.

FIGS. 7A-7D show a clear difference in the density and pore size between 3 mol % Y₂O₃-doped ZrO₂ (3Y-TZP) ˜40 nm particles (3Y-TZP nanoparticles without polymer, as received from the manufacturer) (FIGS. 7A-7B) and 60 wt % 3Y-TZP porous membranes (polymer-templated 3Y-TZP porous membranes, FIGS. 7C-7D), both after sintering cycle. With the sintered 3Y-TZP nanoparticles (FIGS. 7A and 7B) the pores that remain after sintering are smaller and the material is much denser than the porous material that results from the process described herein (FIGS. 7C and 7D). The sintered 3Y-TZP nanoparticles remain as spheres (as shown in the magnified view of FIG. 7B), which may be agglomerates of the particles as processed by the manufacturer using such methods as milling, sieving, etc. to produce spheres.

In contrast, sintered polymer-templated 3Y-TZP porous membranes formed by methods described herein (FIG. 7C and a zoomed-out view FIG. 7D) have a pore structure that is more open and the spherical agglomerates appear broken, fragmented, disintegrated, dispersed, etc., which suggests a homogeneous dispersion of 3Y-TZP in the pre-polymer slurry.

Metal oxide/polymer ratios ranging between 50-70% wt % metal oxide were investigated; different metal oxides were studied including Al₂O₃ (90 nm particles), CeO₂ (20-30 nm particles), 8 mol % Y₂O₃ fully stabilized ZrO₂ (8YSZ, 600 nm particles) and 3Y-TZP (600 and 40 nm particles).

TABLE 1 Behavior properties of porous ceramic membranes Weight lost of Membrane ceramic after Compo- Diameter KOH Days at long-term KOH Metal sition Density Uptake 400° C. exposure Oxide (%) (%) (%) under N₂ at 400° C.* Al₂O₃ 60 20 68 1 Collapsed CeO₂ 65 30 29 4 — 8YSZ 60 25 63 8 28  3Y-TZP 60 32 62 8 5 (600 nm) 3Y-TZP 60 33 54 8 2 (40 nm) *Weight lost was calculated with respect to the initial weight of membranes, before infilling with potassium hydroxide.

Infilling Porous Support Ceramic Membranes with Potassium Hydroxide

After preparation of the porous support ceramic membranes, the stability and loading of the membranes with molten potassium hydroxide was studied. 5 g of potassium hydroxide pellets were melted in a zirconium crucible at 250° C. Different metal oxide porous membranes pre-weighed were immersed in the molten salt for 10 min, and weighed after infilling. FIGS. 8A-8B show SEM images of the surface of a 60 wt % 3Y-TZP porous membrane infilled with KOH (FIG. 8A) and a fracture edge of a 60 wt % 3Y-TZP porous membrane infilled with KOH (FIG. 8B).

Dual-phase membranes were then placed in a tube furnace and heated at 400° C. under N₂ atmosphere for 2 hours to remove the excess of molten hydroxide and water. Results of the hydroxide uptake for Al₂O₃, CeO₂, 8YSZ, and 3Y-TZP porous membranes are summarized in Table 1.

After testing the porous membranes with different metal oxides infilled with potassium hydroxide, it was observed that the Al₂O₃ based membranes showed the highest hydroxide uptake. However, exposure of the hydroxide infilled Al₂O₃ membranes to 400° C. bent the membranes and removing the hydroxide by dissolving in water the membranes collapsed. In contrast, hydroxide infilled CeO₂, 8YSZ, and 3Y-TZP dual-phase membranes survived after long-term heating at 400° C. It was observed that potassium hydroxide was retained within the porous membranes even after several heating and cooling cycles.

FIGS. 9A through 9E are SEMs of porous membranes formed from metal oxide polymer ratios in ranges between 75 wt % 3 mol % Y₂O₃—ZrO₂ (3Y-TZP) (FIG. 9A), 70 wt % 3Y-TZP (FIG. 9B), 65 wt % 3Y-TZP (FIG. 9C). 60 wt % 3Y-TZP (FIG. 9D), and 55 wt % 3Y-TZP (FIG. 9E). The final heating step in the sintering protocol for these membranes was 1100° C. for 15 hours. From these images, it is visually apparent that the density of the membrane of 75 wt % 3Y-TZP (FIG. 9A) is greater than the density of the membrane of 55% wt % 3Y-TZP (FIG. 9E).

FIGS. 10A-10F are SEM images of porous membranes of 70 wt % with different final heating steps in the sintering process. FIG. 10A (magnified image of FIG. 10B) and FIG. 10B involved a final heating step of 1100° C. for 15 hours. FIG. 10C (magnified image of FIG. 10D) and FIG. 10D involved a final heating step of 1200° C. for 3 hours. FIG. 10E (magnified image of FIG. 10F) and FIG. 10F involved a final heating step of 1300° C. for 5 hours.

TABLE 2 Physical Properties of 3Y—ZrO₂ Structures Formed by Different Sintering Conditions LiNaKOH Meso Pore Bending 3Y—ZrO₂ Density (%) Uptake BET Volume Strength wt % after sinter (wt %) (m²/g) (mm³/g) (Mpa) 1100° C. 15 hrs 75% 42.1 31 5.53 16.18 12.1 ± 0.6  70% 37.9 35.9 5.58 19.47 7.5 ± 0.2 65% 35.4 47.9 5.45 17.58 5.6 ± 0.4 60% 32.1 53.4 5.36 17.67 4.1 ± 0.8 55% 27.1 68.5 5.31 18.63 3.2 ± 0.8 1300° C. 5 hrs 70% 81.7 3.6 65% 72.5 6.8 60% 62.3 9.3 1200° C. 3 hrs 70% 68.4 12.1 65% 58.7 16.4 60% 48.7 19.6

Membrane properties of porous membranes formed from metal oxide polymer ratios in ranges between 55-75% wt % 3 mol % Y₂O₃—ZrO₂ (3Y-TZP) (as shown in SEM images of FIGS. 9A-9E and FIGS. 10A-10F) are shown in Table 2. Looking first at the sintering protocol of 1100° C. for 15 hours (upper set), membrane properties of surface area (BET theory), mesopore volume, and bending strength of membranes formed from 55 wt % to 75 wt % 3Y-TZP were compared. There was greater surface area and increased bending strength in the membranes formed from the higher wt % 3Y-TZP, whereas the mesopore volume did not significantly change between the membranes (Table 2). Different sintering protocols were assessed from porous membranes formed from 60% wt % 3Y-TZP, 65 wt % 3Y-TZP, and 70 wt % 3Y-TZP at 1100° C. for 15 hours (upper set), 1300° C. for 5 hours (middle set), and 1200° C. for 3 hours (bottom set) in Table 2. Higher densities were evident with higher sintering temperatures for shorter amounts of time; however, better uptake of LiNaKOH was evident with membranes formed by sintering at the lower temperature of 1100° C. for 15 hours.

Moreover, as shown in FIG. 11 molten LiNaKOH infilled in porous 70 wt % 3Y-TZP membranes demonstrated chemical stability for at least 1000 hours at 550° C. Over the time period, no significant mass loss was detected in the samples.

Membrane Evaluation in Realistic Conditions

Membrane performance under realistic operating conditions of a porous ceramic structure infilled with molten phase. A custom-designed high-temperature permeation cell was placed into a furnace while different mixtures of feed and sweep gasses were introduced. The sweep gas was humidified by passing pre-heated He through a beaker of boiling water. After passing by the membrane, the sweep gas entered a gas chromatograph to determine the concentration of CO₂.

For evaluation, disc-shaped 3Y-TZP supports were glued into a non-porous alumina “donut” before infilling with the hydroxide mixture, and then sealed into the cells using graphite gaskets. Membranes were tested for leaks before single (100% CO₂) and binary mixed gas (50:50 CO₂/N₂) permeation experiments were performed. CO₂ permeabilities of 51.7×10⁻¹² mol m⁻¹ s⁻¹ Pa⁻¹ and 54.8×10⁻¹² mol m⁻¹ s⁻¹ Pa⁻¹ were obtained for the membrane using single and binary mixed gas, respectively, at 550° C. When the temperature was increased to 650° C., CO₂ permeability increased to 62.9×10⁻¹² mol m⁻¹ s⁻¹ Pa⁻¹ and 79.4×10⁻¹² mol m⁻¹ s⁻¹ Pa⁻¹ for single and binary mixed gas, respectively. Very high CO₂ permeability of 64.9×10⁻¹² mol m⁻¹ s⁻¹ Pa⁻¹ at 650° C. was observed even without humidified sweep gas. Without wishing to be bound by any theory, the high CO₂ permeability seemed to be due to the high concentration of hydroxide in the molten phase.

The molten hydroxide dual-phase membranes have CO₂ permeability that is an order of magnitude higher than the best molten carbonate membrane. It should be noted that the custom permeation cell was limited to a maximum of 1 atm of water vapor. If it were possible to introduce pressurized steam, the permeability might be expected to increase further, especially at lower temperature.

The 3Y-TZP ceramic dual phase system with molten 42.5Li/32.5Na/25K formed by methods described herein demonstrated higher CO₂ permeance and higher CO₂ permeability compared to the conventional dual phase membrane systems.

In Use

Various embodiments of porous ceramic membranes with sorbent solution may function in applications of CO₂ separation and sequestration. In some approaches, the porous ceramic membranes with sorbent solution may be useful in carbon capture and storage (CCS), greenhouse gas reduction, and pollution mitigation.

Porous ceramic structures as described may be used as a support material for a liquid/molten phase that may be used in membrane separation techniques, ion conduction, gas sorption, etc.

Unfilled porous ceramic structures may be used as filtration media and used in specialized filtration applications. In some approaches, the porous ceramic structures may be used as high temperature fire-resistant HEPA filters and pre-filters for nuclear applications.

According to inventive concepts described herein, a porous ceramic structure may be used as a support material for catalysis applications.

Inventive concepts as described herein may be used as material for high temperature sensors. Furthermore, the structures as presented may be optimal for high temperature oxide supports and/or membranes used in electrolytic cells and fuel cells.

Moreover, the porous ceramic structure may function as structural material for growth media for cells, for example, as a structure in liquid for the cells to latch onto during cell growth and culture.

The inventive concepts disclosed herein have been presented by way of example to illustrate the myriad features thereof in a plurality of illustrative scenarios, embodiments, and/or implementations. It should be appreciated that the concepts generally disclosed are to be considered as modular, and may be implemented in any combination, permutation, or synthesis thereof. In addition, any modification, alteration, or equivalent of the presently disclosed features, functions, and concepts that would be appreciated by a person having ordinary skill in the art upon reading the instant descriptions should also be considered within the scope of this disclosure.

While inventive concepts have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the various aspects of the present invention should not be limited by any of the above-described exemplary approaches, but should be defined only in accordance with the following claims and their equivalents. 

What is claimed is:
 1. A product, comprising: a three dimensional ceramic structure having an open cell structure with a plurality of pores, wherein the pores connect through the ceramic structure from one side of the ceramic structure to an opposite side of the ceramic structure, wherein an average diameter of the pores is in a range of about 10 nanometers to about 1000 nanometers.
 2. The product as recited in claim 1, wherein the ceramic structure comprises a Y₂O₃-doped ZrO₂ selected from the group consisting of: tetragonal zirconia polycrystal and yttria fully stabilized zirconia.
 3. The product as recited in claim 1, wherein an average diameter of the pores is in a range of about 50 nanometers to about 500 nanometers.
 4. The product as recited in claim 1, wherein a density of the ceramic structure is in a range of about 20% to about 50% of a density of a solid ceramic form of identical composition.
 5. The product as recited in claim 1, comprising a molten hydroxide in the pores, the molten hydroxide being retained in the pores by capillary action.
 6. The product as recited in claim 5, wherein the molten hydroxide is potassium hydroxide.
 7. The product as recited in claim 5, wherein the molten hydroxide is a mixture comprising of lithium hydroxide, potassium hydroxide, and sodium hydroxide.
 8. The product as recited in claim 5, wherein the ceramic structure has physical characteristics that enable the ceramic structure to retain the molten hydroxide in the pores after exposure to temperatures at about 400 degrees Celsius for at least 100 hours.
 9. The product as recited in claim 5, wherein the ceramic structure has physical characteristics that enable the ceramic structure to retain the molten hydroxide in the pores after at least twenty heating and cooling cycles, wherein the heating is to about 400 degrees Celsius, wherein the cooling is to at least room temperature.
 10. A method of forming a porous ceramic structure, the method comprising: combining metal oxide nanoparticles and a polymer to create a slurry mixture; forming a body of the slurry mixture; curing the formed body for crosslinking the polymer; heating the formed body to a first temperature for removing the polymer from the formed body; and heating the formed body to a second temperature for sintering the formed body of the metal oxide nanoparticles, thereby creating a porous ceramic structure having an open cell structure with a plurality of pores, wherein the pores connect through the ceramic structure from one side of the ceramic structure to an opposite side of the ceramic structure.
 11. The method as recited in claim 10, wherein the slurry mixture comprises metal oxide nanoparticles in a range of about 50 wt % to about 80 wt % of total mixture.
 12. The method as recited in claim 10, wherein the metal oxide nanoparticles comprise a Y₂O₃-doped ZrO₂ selected from the group consisting of: tetragonal zirconia polycrystal and yttria fully stabilized zirconia.
 13. The method as recited in claim 10, wherein the metal oxide nanoparticles have an average diameter in a range of at least about 20 nanometers to about 600 nanometers.
 14. The method as recited in claim 10, wherein the forming the body comprises extruding the slurry mixture into a free-standing shape.
 15. The method as recited in claim 10, wherein the formed body has a curved surface.
 16. The method as recited in claim 10, wherein the body of the slurry mixture is formed by additive manufacturing of the slurry mixture.
 17. The method as recited in claim 16, wherein the additive manufacturing is direct ink writing, wherein an ink is the slurry mixture.
 18. The method as recited in claim 10, further comprising filling the porous ceramic structure with an inorganic base material, wherein the inorganic base material includes a molten hydroxide.
 19. The method as recited in claim 18, wherein the molten hydroxide includes molten potassium hydroxide.
 20. The method as recited in claim 18, wherein the molten hydroxide is a mixture comprising of lithium hydroxide, potassium hydroxide, and sodium hydroxide.
 21. A product, comprising: a three dimensional (3D) ceramic structure having a predefined shape characteristic of formation by additive manufacturing processes, wherein the ceramic structure has an open cell structure with a plurality of pores, wherein the pores connect through the ceramic structure from one side of the ceramic structure to an opposite side of the ceramic structure, wherein an average diameter of the pores is in a range of about 10 nanometers to about 1000 nanometers.
 22. The product as recited in claim 21, wherein the ceramic structure has physical characteristics of formation by casting a ceramic mixture into a mold.
 23. The product as recited in claim 21, wherein the ceramic structure has physical characteristics of formation by extruding a ceramic mixture into a free-standing 3D shape.
 24. The product as recited in claim 21, wherein an average diameter of the pores is in a range of about 10 nanometers to about 500 nanometers.
 25. The product as recited in claim 21, comprising a molten hydroxide in the pores, the molten hydroxide being retained in the pores by capillary action. 